A fixing device fixes an image (toner image) formed on a sheet with heat when the sheet passes through the fixing device. One type of the fixing device detects a temperature of the fixing device (e.g., fixing belt or heating roller) and, based on the detected temperature, controls a temperature of a heating region to be within a target temperature range that is preferable to fix the image. One way of controlling the temperature would be turning on and off a heating unit of the fixing device. However, controlling the temperature only by turning on and off the heating unit would be difficult. It would be desirable to control the temperature to be within the target temperature range in an easier manner.